1. Field of the Invention
This invention relates to zinc dust and more particularly to a new and improved zinc dust, an apparatus and method for preparing such dust, and to a new and improved protective coating composition containing such dust.
2. Description of the Prior Art
U.S. Pat. No. 1,738,910 discloses an exceedingly fine zinc powder containing 90% metallic zinc and composed of spherical particles. U.S. Pat. No. 2,123,300 discloses a zinc dust free of metallic cadmium. A high purity, oxide-free zinc powder of extremely high purity and a spherical particle size distribution such that substantially all of the particles have a size less than about 0.1 micron is disclosed in U.S. Pat. No. 3,065,958. U.S. Pat. No. 3,288,746 discloses a lamellar zinc pigment of about 0.1 to 0.5 micron thickness and of 99.97% purity. Zinc powder having spherical particles is also referred to in U.S. Pat. No. 3,288,746. Zinc powder having a grain size on the order of 20 microns or less is disclosed in U.S. Pat. No. 3,403,229. U.S. Pat. Nos. 3,449,116 and 3,449,117 disclose zinc dust having a metallic zinc content of about 99%, and also disclose that zinc dust having a metallic zinc content greater than 98.5% tends to frit and does not flow readily. U.S. Pat. No. 3,464,906 show zinc pigment in the form of flakes and of particle size below 60 microns, for use in zinc paints destined to be deposited electrophoretically.
One important application of zinc dust is in paints or coating compositions for coating steel strip in coils to provide a sacrificial protective coating on the strip. For this application, it is important that an excessive amount of fine zinc-containing particles not be present in the zinc dust and specifically no more than about 3% by weight of the zinc dust of grain size less than 2 microns should be present inasmuch as the presence of the excessive amount of zinc finer than 2 microns is detrimental. The reason amounts of zinc dust fines appreciably greater than about 3% by weight less than 2 microns in grain size is detrimental in the zinc dust for this application, viz. the coating of the steel strip in coils, is that this portion of the zinc dust is usually the highly oxidized portion of the zinc and it tends to result in a highly viscous zinc paint which produces an unsatisfactory, non-uniform coating on the steel. Further this highly oxidized portion of the zinc dust contains undesirable, relatively high concentrations of metal impurities such as, for instance, compounds of aluminum, magnesium, silicon and copper, which result in undesirable paint films. Additionally the presence of such excessive amounts of zinc fines appreciably greater than 3% less than 2 microns results in a materially decreased electrical conductivity and a materially increased electrical resistivity in the paint film and the art has generally considered the decreased conductivity and increased resistivity to be indicative of an inferior zinc-containing protective coating on steel. Further, it is important that an excessive amount of coarse zinc dust particles not be present in the zinc dust utilized for the coating composition for coating the steel strip, for the reasons in thin paint films the coarse particles protrude above the surface of the film and are easily dislodged and pulled out by abrasive forces leaving "holidays"or voids in the paint film, enabling the corrosive fluid, for instance sea water, to contact and hence corrode the ferrous metal substrate.
To the best of my knowledge, the prior art has never come up with or produced a zinc dust having as little as about 3% by weight or less of the zinc dust grains of grain size less than 2 microns while having about 98% or more of the zinc dust grains of grain size less than 15 microns and having about a 96-98% metallic zinc content.